1. Field of the Invention
The present invention relates to a method to control or regulate the basis weight of a paper or cardboard web in the manufacturing process.
2. Discussion of Background Information
An essential quality factor of a paper or cardboard web is the uniformity of the basis weight of the manufactured web. In the manufacturing process of such a web, several interfering factors may arise that can adversely affect or influence the uniformity of the basis weight both across the machine width and in the longitudinal direction. These interfering factors include, e.g., temperature variations, pressure variations, finishing tolerances, and also include, e.g., errors in the operation or adjustment of the machine in the manufacturing process. To substantially eliminate these negative interfering factors and to obtain a uniform production of the paper or cardboard web, devices and methods for controlling and regulating the basis weight, particularly the cross-direction profile of the basis weight, are used.
Patent document DE 35 35 849 discloses changing the width of an outflow opening of a headbox at certain points along the web width so that the throughput of the material suspension is locally altered. However, if the throughput of the material suspension changes locally at an identical concentration over the machine width, the quantity of solids at this point in the web relative to the web width is influenced, which can change the basis weight at this point on this web strip.
In patent application DE 40 19 593, a device and method for regulating the basis-weight cross-direction profile of the web is disclosed. In accordance with this document, when the basis-weight cross-direction profile of the paper or cardboard web deviates at a particular point in the width of the web, the concentration of the material suspension flow should be adjusted or modified at this point. In order to perform this, the above-noted document discloses a headbox to be sectioned at least partially over the machine width with each section to be provided with an individually adjustable concentration via controlled section flows. The individual adjustment of the concentration of a respective section flow occurs through a regulation of the feed stream conditions of two individual flows of constant but different concentrations. Due to the different solid matter content of the section streams, a change of the basis weight results at the corresponding points in the web width.
In the control method utilized above, the basis-weight cross-direction profile of the paper or cardboard web is measured at the end of the paper or cardboard machine and the aperture at the headbox or the feed stream conditions of the individual streams of different concentration are regulated for respective sections via a control loop.
Such a control method with measuring basis-weight cross-direction profile is also known, for example, from patent application DE 40 05 281 or DE 42 38 037. In the above-mentioned documents, the basis-weight cross-direction profile of the paper or cardboard web is measured at the end of the paper or cardboard machine and the necessary adjustment of actuators is performed via a process control system for sectional regulation of the basis-weight.
Also known, e.g., from the disclosure patent document DE 20 19 975, the disclosure of which is expressly incorporated by reference herein in its entirety, is the technique of determining the actual basis-weight cross-direction profile and the actual basis-weight longitudinal profile on a running web with the aid of two sensors. The sensors measure a diagonal profile and a longitudinal profile, and utilize this information to control the web or regulate the basis-weight of a web.
The above-noted control methods substantially control the cross-direction profiles of the paper or cardboard web. It is also known to utilize, during the manufacturing process of a paper or cardboard web, a control loop that measures the basis-weight of the finished paper or cardboard web, which is generally the mean value of the measured cross-direction profile. Based upon the measured basis weight, the material amount supplied to the screen loop is regulated and the basis-weight of the produced paper or cardboard web is adjusted.
Moreover, in the above-noted prior art, the known regulation of the basis weight has a relatively sluggish behavior with long reaction times.
In accordance with the present invention, known paper or cardboard machines and methods can be enhanced to provide a regulation of the basis weight with a shorter reaction time than the prior art devices. In this manner, short wavelength variations in the manufactured paper or cardboard web can be controlled relative to the running time of the paper or cardboard web through the paper or cardboard machine.
The present invention is directed to a paper or cardboard machine that regulates the basis weight of a produced paper or cardboard web during the manufacturing process. The paper or cardboard machine includes at least one headbox for a machine-wide distribution of a material suspension either onto a screen or between two screens, at least one material suspension supply coupled to the headbox with at least one concentration, and at least one initial control loop (I). The at least one initial control loop (I) includes at least one actuator for adjusting the at least one concentration of the at least one supplied material suspension, at least one device for determining a basis weight of the finished web, at least one concentration modifier for modifying the at least one concentration of the at least one material suspension supplied to the headbox, and at least one device for adjusting the at least one actuator for adjusting the at least one concentration of the at least one material suspension supplied. The paper or cardboard machine also includes at least one second control loop (II) for further regulating the concentration of the material suspension supplied to the headbox.
Accordingly, the present invention improves the prior art paper or cardboard machine with regulation of the basis weight of a paper or cardboard web in the manufacturing process by utilizing at least a second control loop for regulating the concentration of the material suspension supplied to the headbox.
A further feature of the paper or cardboard machine of the present invention is that the second control loop includes at least one device for either the direct or indirect determination of the basis weight of the resulting web.
In accordance with another embodiment of the present invention, the device for either the direct or indirect determination of the basis weight of the resulting web is positioned in a wet area of the paper or cardboard machine. This positioning may be utilized to obtain measurement directly behind the headbox jet, e.g., with a radiometric sensor. Further, the ability to measure the height of the material suspension on an extended screen is possible. This measurement may advantageously occur, e.g., preferably behind a first roll and prior to a first draining unit so as to substantially avoid influences that arise through the draining unit. However, if it is necessary, e.g., for reasons of space requirements or other prevailing conditions, measuring may be alternatively performed after the first draining unit. Further, even in a double-screen environment, the basis weight of the resulting web can be determined in the wet section, e.g., with the radiometric measurement.
In another embodiment of the present invention, the headbox can have two material supplies provided with different material concentrations, e.g., CL and CH. These material supplies may be formed so that at least one of the two material supplies is divided at the headbox into section streams that are to be individually controlled. In this regard, reference is made to the above-noted prior art documents, which disclose various embodiments of headboxes that are controllable for consistency.
In accordance with another aspect of the present invention, one of the two material supplies may be coupled to a thinning or thickening fluid supply that is to be regulated by the second control loop. For example, the thinning supply may be screen water, and the thickener supply may include additional thick matter to be controllably introduced into one of the two material supplies. Other thinning or thickening fluids can also be used, e.g., clarified water, clarified filtrate, or material suspension having a higher concentration.
In another embodiment of the present invention, if fibrous materials or ash content of the forming paper or cardboard are measured separately by the second sensor located within the wet section of the paper or cardboard machine or if information related to these properties is received via the paper or cardboard material consistency sensor, the corresponding, individual deficiencies of fibrous materials or ash can be supplied in a targeted manner so as to compensate for the measured deficiency. Accordingly, two different sensors may be utilized to measure fibrous materials and ash contend in the resulting web in accordance with the present invention without departing from the scope of the invention.
In accordance with the method of the present invention, the prior art process for regulating the basis weight of a paper or cardboard web, e.g., in which the basis weight of the web is determined at the end of the manufacturing process, a variation of the determined value is calculated from a predetermined target value, and the concentration of the material suspension supplied to the headbox is adjusted to correct the calculated deviation, is improved such that the basis weight of the resulting web is measured either directly or indirectly in a region located before the drying section and the measured basis weight is supplied to a second control loop.
In accordance with another alternative embodiment of the present invention, the second control loop regulates the concentration of the material suspension supplied to the headbox.
The process of the present invention further provides that the concentration of the material suspension supplied to the headbox is regulated via the second control loop by supplying either a thinning or thickening fluid in variable amounts to the material suspension supply to the headbox.
Another embodiment of the process includes two suspension streams having different concentrations. The two suspension streams are supplied to the headbox and the regulation of the concentration of at least one of the material supplies is influenced by the second control loop. In this manner, actuators, e.g., throttle valves or speed-regulated pumps, may be controlled in accordance with the measured requirements.
In accordance with another embodiment of the present invention, the second control loop may engage in the existing material density regulation with a consistency-controlled headbox throughout the machine. In this manner, all actuators over the machine width that are provided for regulation of a cross-direction profile may be additionally influenced by the second control loop to provide a uniform machine-wide adjustment of these actuators. This embodiment has the advantage that additional actuators are not necessary and that a simple and uncomplicated enhancement of a known paper or cardboard machine with a consistency-controlled headbox may be effected by adding the second control loop with a second sensor.
Of course, the above-noted features and further features to be explained below are not only applicable in each of the combinations given, but also in other combinations or in isolation, without departing from the scope of the invention.
Accordingly, the present invention is directed to a paper or cardboard machine for regulating a basis weight of a paper or cardboard web during a manufacturing process. The paper or cardboard machine includes at least one headbox that extends across a machine width. The at least one headbox is adapted to provide a machine-wide distribution of a material suspension onto one of a screen and between two screens. The machine also includes at least one material suspension supply coupled to the at least one headbox and adapted to supply a material suspension to the at least one headbox. The at least one material suspension supply has at least one concentration. At least one first control loop is provided that includes at least one actuator being adapted to adjust the at least one concentration of the at least one material suspension supply, at least one detector for determining a basis weight of a finished web, at least one supply being adapted to modify the at least one concentration of the at least one material suspension supply, and at least one device that adjusts the at least one actuator. In this manner, the at least one concentration of the at least one material suspension supply is adjusted. At least one second control loop is provided that is adapted to regulate the concentration of the material suspension supply.
In accordance with a further feature of the present invention, at least one sensor is adapted to determine the basis weight of a resulting web one of directly and indirectly, and the second control loop is coupled to at least one sensor. Further, the at least one sensor is positioned in a wet section of the paper or cardboard machine.
In accordance with another feature of the present invention, at least one second actuator is adapted to regulate an amount of thick matter to be supplied to the headbox, and the second control loop is coupled to at least one second actuator.
In accordance with another feature of the present invention, the at least one material suspension supply includes two material supplies having a high and low material concentration, respectively. Further, one of the two material supplies includes a regulatable input adapted to receive one of a thickening and a thinning fluid, and the regulatable input is coupled to and regulated by the second control loop. Still further, the one of the two material supplies with the low material concentration has the regulatable input. Further still, the one of the two material supplies with the high material concentration has the regulatable input.
In accordance with a still further feature of the present invention, at least one of the two material supplies includes a plurality of branches coupled to the headbox, and each branch is adapted to control an amount of flow.
The present invention is directed to a method for regulating a basis weight of a paper or cardboard web in the manufacturing process. The method includes determining a basis weight at an end of the manufacturing process, calculating a deviation of the determined basis weight from a predetermined target value, adjusting a concentration of a material suspension supplied to a headbox via a first control loop such that the calculated deviation is corrected, one of directly and indirectly measuring a basis weight of a resulting web in a region upstream, with regard to a web travel direction, a drying section, and supplying a signal related to the measured basis weight to a second control loop.
In accordance with a further feature of the present invention, further including regulating a concentration of the material suspension supplied to the headbox via the second control loop. The regulating of the concentration of the material suspension supplied to the headbox via the second control loop includes variably supplying an amount of one of a thinning and thickening fluid to the material suspension supplied to the headbox.
In accordance with another feature of the present invention, the material suspension supplied to the headbox is supplied in two separate supplies having a high and low concentration, and the regulating of the concentration of the material suspension supplied to the headbox via the second control loop includes regulating at least one of the two separate supplies via the second control loop.
In accordance with still another feature of the present invention, further including filtering the measured basis weight signal to suppress low-frequency variations prior to the second control loop.
In accordance with a still further feature of the present invention, further including filtering the determined basis weight signal to suppress high-frequency variations prior to the first control loop.
In accordance with another feature of the present invention, further including exchanging information between the first control loop and the second control loop such that conflicting adjustment and regulation of the material suspension concentration is substantially avoided.
The present invention is also directed to a machine for producing a web. The machine includes a headbox, a wet section having at least one screen, and a material suspension coupled to the headbox. The headbox is adapted to supply the material suspension into the wet section of the machine. At least one sensor is positioned within the wet section to one of directly and indirectly determine a basis weight of a resulting web, and at least one control loop is adapted to regulate a concentration of the material suspension supplied to the wet section.
In accordance with a further feature of the present invention, a second sensor is positioned within the headbox to determine a material consistency of the material suspension and being coupled to the at least one control loop.
In accordance with a still further feature of the present invention, a thinning medium supply and a thickening medium supply, the thinning and thickening medium supplies being coupled to the material suspension through regulatable devices, and the regulatable devices are adapted to be regulated by the control loop.
In accordance with yet another feature of the present invention, at least one detector for determining a basis weight of a finished web, at least one concentration supply is adapted to modify the concentration of the material suspension, at least one additional control loop, at least one actuator is coupled to the at least one concentration supply. The at least one actuator is adapted to be controlled by the at least one additional control loop.
Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawing.